Porous materials such as membranes are used as filtration devices to eliminate undesirable contaminants from useful end products. Pharmaceutical, food processing and biotechnology industries require that the filtration devices comply with certain performance criteria. Integrity testing provides a means for ensuring that the device meets the desired performance criteria. With membrane filtration devices, integrity testing assures that the membrane is free of defects of a certain size, the presence of which would compromise device performance by allowing the end product to be contaminated.
One known integrity test method is the gas-liquid diffusion test, this test measures the flow of a gas through a liquid-wetted porous filter when the filtration device is pressurized on one side with the gas at a set pressure below the bubble point of the filter. If the device is free of defects, the measured flow rate of the gas will be relatively low at a given pressure, because the measured flow is primarily diffusive flow and not bulk gas flow. A device with defects however, will exhibit relatively large, bulk gas flow rates attributable to gas flowing through the defects in the porous filter.
For large multi-round housings with large porous filter areas, the diffusive flow through the filter material is often so high, however, that it cannot be distinguished from bulk flow attributable to small defects. Similarly, devices using extremely thin membranes such as those described in U.S. Pat. No. 7,306,729, have inherently high diffusive flow through the membrane which may make it difficult to recognize flow due to a defect. Therefore, the utility of the gas-liquid diffusion test for large filtration devices is limited. There exists a need for an integrity test method which provides increased sensitivity.